The ontogeny of glutamine uptake by jejunal brush border membrane vesicles was studied in suckling and weanling rats and compared with the data obtained from previous studies done on adult rats in our laboratory. Glutamine uptake represented transport into the intravesicular space rather than mere binding into the membrane as evident by osmolality study. The process of glutamine uptake was temperature dependent suggesting a carriermediated process with a pH optimum at 7.0. Glutamine uptake was driven by Na+and K+gradient in both suckling and weanling rats. Both processes exhibited saturation kinetics and were inhibited by other neutral amino acids suggesting the presence of Na+-dependent neutral brush border system and Na+-independent (L)-like system. The Vmaxof Na+-dependent and Na+-independent processes were significantly greater in suckling rats with Vmaxof 4.9 ± 0.36 nmol·mg protein-1.7 s-1compared to weanling rats with Vmaxof 2.4 ± 0.2 nmol·mg protein-1. 7 s-1and adult rats with Vmaxof 0.70 nmol·mg protein-1-7 s-1. The greater Vmaxin suckling rats is also evident when the kinetic parameters are analyzed by subtracting the sodium-dependent uptake values from the sodium-independent values. Vmaxof 1.59 ±0.3 and 0.76 ± 0.01 nmol·mg protein-17 s-1in suckling and weanling rats, respectively, p<0.01. Km values were not different at 2.5 ± 0.6 and 3.5 ± 0.6 mM, respectively). The data suggest that the activity and/ or the number of transporters are greater during the period of active growth and development. We conclude that glutamine transport in brush border membrane vesicles undergoes age-dependent changes with greater maximal capacity to transport glutamine in the suckling period.